Critical activating nucleus

The critical activating nucleus forms as follows A lattice defect at the surface (e.g., a dislocation, a step, or a kink) can first trap an electron, then one or two interstitial silver ions, then one or two more electrons, and so on, reacting with electrons and ionic defects alternately ... 

Here AG is the critical activation free energy which, for a spherical nucleus and isotropic liquid-crystal interfacial tension (o) and the Hquid-crystal free energy difference per unit volume (AG ), is given by,... 

Nucleation is the initial process leading to the formation of a new phase. Classical nucleation theory [11-13] describes homogeneous nucleation as the breakdown of a metastable state that occurs at a critical activation energy, which is achieved at a critical subcooling (in melts) or supersaturation (in solution). The homogeneous nucleus is conceived of as an aggregate of critical size in unstable equilibrium with the parent phase. At concentrations below the critical level the cluster grows or dissociates reversibly. 

For homogeneous nucleation in condensed systems diffusion to the phase boundary becomes an important factor and A/, the energy of activation for this diffusion, must be added to AF, the free energy of formation of a critical size nucleus, AF = 6na l3 AGy. A detailed treatment of this problem has been given by Turnbull and Fisher ( ). 

The relative magnitude of these two activation free energies determines the size and shape of the critical nucleus, and hence of the resulting crystal. If sliding diffusion is easy then extended chain crystals may form if it is hard then the thickness will be determined kinetically and will be close to lmin. The work so far has concentrated on obtaining a measure for this nucleus for different input parameters and on plotting the most likely path for its formation. The SI catastrophe does not occur because there is always a barrier against the formation of thick crystals which increases with /. 

According to Eq. (23), the critical pore radius r greatly decreases with increasing electrode potential. It is seen that above a certain critical potential AE b the active barrier as well as the critical pore radius decreases steeply with anodic potential. This critical potential AE is the lowest potential of pore formation and below this potential the passive film is stable against electrocapillary breakdown because of an extremely high activation barrier and the large size of pore nucleus required. 

Figure 17. Energy for the nucleation of a surface film on metal electrode. M, metal OX, oxide film EL, electrolyte solution. Aj is the activation barrier for the formation of an oxide-film nucleus and rj is its critical radius. 7 a is the interfacial tension of the metal-electrolyte interface, a is the interfacial tension of the film-electrolyte interface. (From N. Sato, J. Electro-chem. Soc. 129, 255, 1982, Fig. 5. Reproduced by permission of The Electrochemical Society, Inc.)...

Investigation of the neurochemical substrates for the psychostimulant effects of MDMA suggests a role for the mesolimbic dopamine system. Destruction of dopamine terminal fields in the nucleus accumbens significantly attenuated the locomotor activation produced by MDMA. A similar blockade of amphetamine-induced locomotor hyperactivity is known and was observed following amphetamine injection in these same rats. Such results support the hypothesis that at least one component of MDMA-induced hyperactivity is dopamine mediated and suggest that mesolimbic dopamine specifically is the critical substrate. In this way, MDMA resembles other classical psychostimulants like amphetamine and cocaine. Interestingly, evidence for functional cross-sensitization was suggested in the study in which an injection of amphetamine followed MDMA injection. 

Since Jo and C are proportional to the diffusion coefficient (D) and activation free energy for formation of a critical nucleus (AG ), respectively, it is concluded that the Z dependence of J is mainly determined by the diffusion process of the polymer chain and not by the formation process of a critical nucleus. 

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